1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly, to a reference voltage generator which can generate a reference voltage which is less dependent on temperature.
2. Description of the Related Art
A reference voltage is a voltage which is referenced when determining a logic level of data. That is, data is compared with a reference voltage. Thereafter, if the voltage of the data is determined to be lower than the reference voltage, the data is determined as being logic low. Otherwise, the data is determined as being logic high. Therefore, when a reference voltage changes, a logic level of data which is compared with the reference voltage may not be accurately determined. In addition, a reference voltage can be used to generate an internal power supply voltage in a memory device such as a dynamic random access memory (DRAM).
A reference voltage must be uniform regardless of operating conditions, temperature variations, and power supply voltage variations. A variety of circuits for generating a reference voltage have been developed. An example of such circuits is disclosed in U.S. Pat. No. 5,309,083 A.
FIG. 1 is a circuit diagram of a conventional reference voltage generator. Referring to FIG. 1, the conventional reference voltage generator includes a preliminary reference voltage generation unit 11 which generates a preliminary reference voltage VREFP, a reference voltage generation unit 13 which generates a reference voltage VREF, and a voltage adjustment unit 15 which adjusts the preliminary reference voltage VREFP.
The preliminary reference voltage generator 11 includes a plurality of resistors RS and R11 and a plurality of NMOS transistors NM1 and NM2. The reference voltage generation unit 13 includes a resistor R21 and a plurality of NMOS transistors NM3 and NM4. The voltage adjustment unit 15 includes a PMOS transistor PM1.
The preliminary reference voltage VREFP, which is generated by the preliminary reference voltage generation unit 11, may be indicated by Equation (1):
                                                        VREFP              =                              Vtp                +                                  (                                      Io                    ×                                          R                      on                                                        )                                                                                                        =                              Vtp                +                                  (                                                            Vtp                                              R                        ⁢                                                                                                  ⁢                        11                                                              ×                                          R                      on                                                        )                                                                                                        =                              Vtp                +                                  (                                                                                    R                        on                                                                    R                        ⁢                                                                                                  ⁢                        11                                                              ×                    Vtp                                    )                                                                                                        =                              Vtp                ×                                  (                                      1                    +                                                                  R                        on                                                                    R                        ⁢                                                                                                  ⁢                        11                                                                              )                                                                                        (        1        )            where Io indicates a current flowing through the resistor R11 in the preliminary reference voltage generation unit 11, Ron indicates the sum of the resistances of the NMOS transistors NM1 and NM2 in the preliminary reference voltage generation unit 11, and Vtp indicates a threshold voltage of the PMOS transistor PM1 of the voltage adjustment unit 15.
The reference voltage VREF, which is generated by the reference voltage generation unit 13, may be indicated by Equation (2):
                    VREF        =                  VREFP          ×                                    R                              t                ⁢                                                                  ⁢                on                                                                    R                                  t                  ⁢                                                                          ⁢                  on                                            +                              R                ⁢                                                                  ⁢                21                                                                        (        2        )            where Rton indicates the sum of the resistances of the NMOS transistors NM3 and NM4 in the reference voltage generation unit 13.
The preliminary reference voltage VREFP, which is generated by the preliminary reference voltage generation unit 11, is inversely proportional to temperature. In order to compensate for this characteristic, the reference voltage VREF, which is generated by the reference voltage generation unit 13, is designed to be proportional to temperature. As a result, the reference voltage VREF is relatively robust against temperature variations.
However, the conventional reference voltage generator cannot adequately adjust the reference voltage VREF and the dependency of the reference voltage VREF on temperature at the same time. That is, if Rton is set high to increase the reference voltage VREF, the dependence of the reference voltage VREF on temperature increases. On the other hand, if Rton is set low to lower the dependence of the reference voltage VREF on temperature, the reference voltage VREF decreases.